Digital radiography detector image readout process

ABSTRACT

A radiographic detector acquires a first partial exposed image signal during an image readout of each of the rows of photosensors, one row at a time. A first scan of each row includes measuring the charge delivered to each cell of the rows, including some rows having partial charge and other rows having full charge, and obtaining a first null image signal during the scan. A second scan includes measuring remaining charge delivered to those rows having partial charge. The null image signal data is subtracted from a sum of the first two scans.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/487,189, filed Sep. 16, 2014, in the name of Hawver et al., entitledDIGITAL RADIOGRAPHY DETECTOR IMAGE READOUT PROCESS, which claims thebenefit of U.S. Provisional Application U.S. Ser. No. 61/879,182,provisionally filed on Sep. 18, 2013, entitled “DR DETECTOR IMAGEREADOUT PROCESS”, in the names of Hawver, et al., which is herebyincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates generally to the field of medical imaging, and inparticular to digital radiographic imaging and more particularly toapparatus and methods for enhancing the signal integrity of the imagereadout operation of a digital radiography detector.

BACKGROUND OF THE INVENTION

Stationary and mobile radiographic imaging equipment is employed inmedical facilities to capture x-ray images on an x-ray detector. Suchmedical x-ray images may be captured using various techniques such ascomputed radiography (CR) and digital radiography (DR) in radiographydetectors.

A related art DR imaging panel acquires image data from a scintillatingmedium using an array of individual sensors, arranged in a row-by-columnmatrix, in which each sensor provides a single pixel of image data. Eachpixel generally includes a photosensor and a switching element that maybe fabricated in a co-planar or a vertically integrated manner, as isgenerally known in the art. In these imaging devices, hydrogenatedamorphous silicon (a-Si:H) is commonly used to form the photodiode andthe thin-film transistor switch needed for each pixel. In one knownimaging arrangement, a frontplane includes an array of photosensitiveelements, and a backplane includes an array of thin-film transistor(TFT) switches.

However, there is a need for improvements in the consistency and qualityof medical x-ray images, particularly when obtained by an x-rayapparatus designed to operate with a-Si DR detectors. There is also aneed for detection of an x-ray exposure event without requiring that anx-ray exposure be delayed until the DR detector is ready, such as havingexternal hardware connections that link to and hold off the x-ray sourcecontrol electronics. Further, there is a need for detection ofextraneous signals produced by nearby sources of low frequency magneticfields before an x-ray exposure and an imaging readout operation isinitiated.

BRIEF DESCRIPTION OF THE INVENTION

It would be advantageous to provide a method and apparatus to detectextraneous low frequency magnetic fields near image readout circuitrybefore an x-ray exposure and an imaging readout operation is performed.This capability provides the benefit of alerting the operator of a DRsystem to this noise condition and minimizes radiation exposure to apatient that produces DR images not meeting clinical diagnosticstandards. It would also be advantageous to provide a method for theremoval of image artifacts generated when the image readout process runsconcurrently or overlaps with the x-ray beam exposure.

In one embodiment, an area x-ray detector is disclosed comprising anumber of electrically-chargeable photosensitive cells arranged in rowsand columns. Electric circuits, or charge integrators, are attached tothe cells of each column by controllable row selecting switches toprovide a reading of charge delivered to the cells of each column.Acquisition control electronic circuitry is programmed to acquire afirst partial exposed image signal during a readout of each of the rowsof cells one row at a time, a first scan of a current row includingmeasuring the total charge delivered to each cell of the row using theelectric circuits, resetting the electric circuits, restoring the chargeof each cell of the current row by means of the electric circuits;acquiring a first null image signal during the readout of each of therows of cells one row at a time, and acquiring a second scan of thecurrent row including measuring the total charge delivered to each cellof the row by means of the electric circuits, and resetting the electriccircuits.

In another embodiment, there is disclosed a method of operating an areax-ray detector that includes a plurality of electrically-chargeablephotosensitive pixels arranged in rows and columns with chargeintegrator circuits attached to the pixels of each column to provide areading of charge delivered to the pixels of each column. The methodcomprises acquiring a pair of signals during a dual scan of each of therows of a portion of the pixels, one row at a time. The dual scancomprises a first scanning of each row including enabling the pixels ofa current row for a first predetermined time period, measuring the totalcharge delivered to each pixel of the row by means of the chargeintegrator circuits, outputting the total charge to form a row of afirst partial image signal, and a resetting of the charge integratorcircuits and the pixels of the current row. A second scanning of eachrow includes disabling the pixels of the current row, measuring thecharge delivered to each column of said each pixel of the current row bymeans of the charge integrator circuits, outputting the charge deliveredto each column of said each cell to form a null image signal, andresetting the charge integrator circuits.

In another embodiment, a computer implemented method comprises scanninga charge level in a two dimensional array of pixels and recording in afirst portion of electronic memory the charge level of the scannedpixels. The two dimensional array of pixels is exposed to radiographicradiation during the step of scanning which generates charges in thearray of pixels such that scanning results in at least one of a firstsubset of the scanned pixels having a partial image charge. A secondsubset of the scanned pixels have a full image charge. The pixels arerescanned and recorded in a second portion of the electronic memory, andthen the charge recorded in the first portion of the electronic memoryand the charge recorded in the second portion of the electronic memoryare summed. An advantage that may be realized in the practice of somedisclosed embodiments of the DR imaging system and methods is improvedreadout of DR images by correcting artifacts caused by extraneousmagnetic fields.

This brief description of the invention is intended only to provide abrief overview of subject matter disclosed herein according to one ormore illustrative embodiments, and does not serve as a guide tointerpreting the claims or to define or limit the scope of theinvention, which is defined only by the appended claims. This briefdescription is further provided to introduce an illustrative selectionof concepts in a simplified form that are described below in thedetailed description, and is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter. The claimed subject matter is not limited to implementationsthat solve any or all disadvantages noted in the background.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the features of the invention may beunderstood, a detailed description of the invention is disclosed withreference to certain embodiments, some of which are illustrated in theaccompanying drawings. It is to be noted, however, that the drawingsillustrate only certain embodiments of this invention and are thereforenot to be considered limiting of its scope, for the scope of theinvention encompasses other equally effective embodiments. For example,the summary descriptions above are not meant to describe individualseparate embodiments whose elements are not interchangeable. In fact,many of the elements described as related to a particular embodiment maybe used together with, and possibly interchanged with, elements of otherdescribed embodiments. Many changes and modifications may be made withinthe scope of the present invention without departing from the spiritthereof, and the invention includes all such modifications. The drawingsbelow are intended to be drawn neither to any precise scale with respectto relative size, angular relationship, relative position, or timingrelationship, nor to any combinational relationship with respect tointerchangeability, substitution, or representation of a requiredimplementation. In the drawings, like numerals are used to indicate likeparts throughout the various views. Thus, for further understanding ofthe invention, reference may be made to the following detaileddescription, read in connection with the drawings in which:

FIG. 1 is a perspective view of an exemplary radiographic imagingsystem.

FIG. 2 is a schematic diagram of a portion of an exemplary imaging arrayfor a DR detector used in the exemplary radiographic imaging system ofFIG. 1.

FIG. 3 shows a perspective view of an exemplary portable wireless DRdetector.

FIG. 4 is an exemplary cross-sectional view along section line A-A ofthe portable wireless DR detector of FIG. 3.

FIG. 5 is a diagram showing an exemplary pixel cell and selectedconstituent components.

FIGS. 6A-B illustrate an exemplary pixel cell showing the generation ofextraneous signals.

FIG. 7 is a diagram showing an exemplary readout process for an imagereadout operation in a DR detector.

FIG. 8 is a diagram showing an exemplary image readout process using aninterleaved null row read process for a DR detector.

FIG. 9 is a diagram showing exemplary waveforms and image sets producedby an exemplary image readout procedure.

FIGS. 10-11 illustrate an exemplary reconstruction using two image datasets.

DETAILED DESCRIPTION OF THE INVENTION

During an imaging readout operation performed by a DR detector there mayexist unwanted extraneous signals that affect the DR detector's readoutoperation by introducing data errors that result in degraded imagequality when the image data is finally processed. The extraneous signalsmay originate from noise sources external to the detector or fromsources within the detector housing. Extraneous signals may also beproduced during the readout operation if the image readout occursconcurrently with an activation of the x-ray source. Extraneous magneticfields may be generated by the DR system and related equipment in animaging room which may cause parasitic effects in the readout circuitryor on the pixels in the photosensor array.

One type of external extraneous signal commonly found to interfere withDR detector image readout operations is caused by low frequency magneticfields in the range of about one kilohertz up to hundreds of kilohertz.These magnetic fields may be produced by electrical equipment in closeproximity to the DR detector. Typically, these noise inducing magneticfields are generated by components such as inductors or AC motors thatemit magnetic flux. Another source of extraneous noise includes powersupplies that generate high voltages. These power supplies are oftenrequired by automatic exposure control hardware used with DR detectors.

Extraneous noise affecting operations of the DR detector find entrypoints into the DR detector via parasitic capacitance intrinsic to thepixel array of the DR detector. A DR detector's readout operation may beperformed after an x-ray source has exposed the patient and detector tox-ray radiation for a fixed exposure period predetermined and configuredby an operator. The corresponding DR detector integration period, whichis concurrent with the x-ray source “on time”, may be configured toterminate after the x-ray source is turned off, because the imagereadout process normally follows the DR detector integration period. Aportion of an image readout process occurring during the x-ray sourceexposure period is susceptible to the noise signals caused by the x-raysource.

There may be occasions when it is desirable to perform the image readoutconcurrently with the x-ray exposure. In this case the image readoutfrom the DR detector may be initiated before the x-ray source exposureprocess has begun. The image readout process may run continuously untilall image frames are acquired and stored.

The start of an x-ray exposure may be detected by image processingsoftware running concurrently with the readout process where the imageprocessing software tests each read out image row for increased signalintensity. After the start of the x-ray beam exposure is detected, therow by row image readout continues until the signal level returns to apre-exposure level at about zero. After the x-ray beam exposure periodhas finished, the image readout process continues for at least one moreimage readout cycle to obtain a final ‘image free’ frame (unexposed) ofdata called a dark image, or lag image, that is used to adjust andcorrect previous image data frames. When all collected image frames havebeen stored in an image buffer, which may include an image bufferinternal to the DR detector comprising electronic memory locations forstoring several image data frames, a post image processing function isperformed on the buffered image frames to produce the final image. Thismethod of image readout for a DR detector has the benefit of providingasynchronous image readout of an x-ray exposure event without the needfor invasive external hardware connections linking to and holding offthe x-ray source control electronics until the DR detector system isready for an x-ray exposure. This image readout method, however, causesimage artifacts induced in part by parasitic capacitance and x-ray beamexposure that generate leakage current during the readout method.

FIG. 1 is a perspective view of a digital radiographic (DR) imagingsystem 10 that includes a generally planar DR detector 40 (shown withouta housing for clarity of description), an x-ray source 14 configured togenerate radiographic energy (x-ray radiation), and a digital monitor 26configured to display images captured by the planar DR detector 40,according to one embodiment. The planar DR detector 40 may include a twodimensional array 12 of photosensitive detector cells 22 (photosensors),arranged in electronically addressable rows and columns. The planar DRdetector 40 may be positioned to receive x-rays 16 passing through asubject 20 during a radiographic energy exposure, or radiographic energypulse, emitted by the x-ray source 14. As shown in FIG. 1, the digitalradiography (DR) imaging system 10 may use an x-ray source 14 that emitscollimated x-rays 16, e.g. an x-ray beam, selectively aimed at andpassing through a preselected region 18 of the subject 20. The x-rays 16may be attenuated by varying degrees along its plurality of raysaccording to the internal structure of the subject 20, which attenuatedrays are detected by the two-dimensional array 12 of photosensitivedetector cells 22. The planar DR detector 40 is positioned, as much aspossible, in a perpendicular relation to a substantially central ray 17of the plurality of x-rays 16 emitted by the x-ray source 14. Thetwo-dimensional array 12 of individual photosensitive detector cells(pixels) 22 may be electronically read out (scanned) by their positionaccording to column and row. As used herein, the terms “column” and“row” refer to the vertical and horizontal arrangement of thephotosensitive detector cells 22 and, for clarity of description, itwill be assumed that the rows extend horizontally and the columns extendvertically. However, the orientation of the columns and rows isarbitrary and does not limit the scope of any embodiments disclosedherein. Furthermore, the term “subject” may be illustrated as a humanpatient in the description of FIG. 1, however, a subject of a DR imagingsystem 10, as the term is used herein, may be a human, an animal, aninanimate object, or a portion thereof.

In one exemplary embodiment, the rows of photosensitive detector cells22 may be scanned one or more at a time by electronic scanning circuit28 so that the exposure data from the two-dimensional array 12 may betransmitted to electronic read-out circuit 30. Each photosensitivedetector cell 22 may independently store a charge proportional to anintensity, or energy level, of the attenuated radiographic radiation, orx-rays 16, received and absorbed in the cell. Thus, each photosensitivedetector cell 22, when read-out, provides information defining a pixelof a radiographic image 24, e.g. a brightness level or an amount ofenergy absorbed by the pixel, that may be digitally decoded byacquisition control and image processing unit 34 and transmitted to bedisplayed by the digital monitor 26 for viewing by a user. An electronicbias circuit 32 is electrically connected to the two-dimensional array12 to provide a bias voltage to each of the photosensitive detectorcells 22.

Each of the electronic bias circuit 32, the electronic scanning circuit28, and the electronic read-out circuit 30, may communicate with anacquisition control and image processing unit 34 over a connected cable(wired), or the planar DR detector 40 may be equipped with a wirelesstransmitter to transmit radiographic image data wirelessly to theacquisition control and image processing unit 34. The acquisitioncontrol and image processing unit 34 may include a processor andelectronic memory (not shown) to control operations of the planar DRdetector 40 as described herein, including control of circuits 28, 30,and 32, for example, by use of programmed instructions. The acquisitioncontrol and image processing unit 34 may also be used to controlactivation of the x-ray source 14 during a radiographic exposure,controlling an x-ray tube electric current magnitude, and thus thefluence of x-rays 16 in x-ray beam, and the x-ray tube voltage, and thusthe energy level of the x-rays 16 in x-ray beam.

The acquisition control and image processing unit 34 may store aplurality of data frames received from the planar DR detector andtransmit image (pixel) data to the digital monitor 26, based on theradiographic exposure data received from the two-dimensional array 12 ofphotosensitive detector cells 22 in the planar DR detector 40.Alternatively, acquisition control and image processing unit 34 mayprocess the image data and store it, or it may store raw unprocessedimage data, in local or remotely accessible memory.

With regard to a direct detection embodiment of planar DR detector 40,the photosensitive detector cells 22 may each include an x-ray sensingelement sensitive to x-rays 16, i.e. it absorbs x-rays 16 and generatesan amount of charge carriers in proportion to a magnitude of theabsorbed x-ray energy. A switching element may be configured to beselectively activated to read out the charge level of a correspondingx-ray sensing element. With regard to an indirect detection embodimentof a planar DR detector 40, photosensitive detector cells 22 may eachinclude a sensing element sensitive to light rays in the visiblespectrum, i.e. it absorbs light rays and generates an amount of chargecarriers in proportion to a magnitude of the absorbed light energy, anda switching element that is selectively activated to read the chargelevel of the corresponding sensing element. A scintillator, orwavelength converter, is disposed over the light sensitive sensingelements to convert incident x-ray radiographic energy to visible lightenergy. Thus, in the embodiments disclosed herein, it should be notedthat the planar DR detector 40 may include an indirect or direct type ofplanar DR detector 40.

Examples of sensing elements used in a two-dimensional array 12 includevarious types of photoelectric conversion devices (e.g., photosensors)such as photodiodes (P-N or PIN diodes), photo-capacitors (MIS),phototransistors or photoconductors. Examples of switching elements usedfor signal read-out include MOS transistors, bipolar transistors andother p-n junction components.

FIG. 2 is a schematic diagram 240 of a portion of a two-dimensionalarray 12 for the planar DR detector 40. The array of photosensor cells212, whose operation may be consistent with the two-dimensional array 12described above, may include a number of hydrogenated amorphous silicon(a-Si:H) n-i-p photodiodes 270 and thin film transistors (TFTs) 271formed as field effect transistors (FETs) each having gate (G), source(S), and drain (D) terminals. In embodiments of planar DR detector 40disclosed herein, such as a multilayer DR detector, the two-dimensionalarray 12 may be formed in a device layer that abuts adjacent layers ofthe DR detector structure. A plurality of gate driver circuits 228 maybe electrically connected to a plurality of gate lines 283 which controla voltage applied to the gates of TFTs 271, a plurality of readoutcircuits 230 may be electrically connected to a plurality of data lines284, and a plurality of bias lines 285 may be electrically connected toa bias line bus or a variable bias reference voltage line 232 whichcontrols a voltage applied to the photodiodes 270. Charge amplifiercircuits 286 may be electrically connected to the data lines 284 toreceive signals therefrom. Outputs from the charge amplifier circuits286 may be electrically connected to a multiplexer 287, such as ananalog multiplexer, then to an analog-to-digital converter (ADC) 288, orthey may be directly connected to the ADC, to stream out the digitalradiographic image data at desired rates. In one embodiment, theschematic diagram of FIG. 2 may represent a portion of a planar DRdetector 40 such as an a-Si:H based indirect flat panel imager.

Incident x-rays 16, or x-ray photons, are converted to optical photons,or light rays, by a scintillator, which light rays are subsequentlyconverted to electron-hole pairs, or charges, upon impacting the a-Si:Hn-i-p photodiodes 270. In one embodiment, an exemplary detector cell222, which may be equivalently referred to herein as a pixel, mayinclude a photodiode 270 having its anode electrically connected to abias line 285 and its cathode electrically connected to the drain (D) ofTFT 271. The bias reference voltage line 232 may control a bias voltageof the photodiodes 270 at each of the detector cells 222. The chargecapacity of each of the photodiodes 270 is a function of its biasvoltage and its capacitance. In general, a reverse bias voltage, e.g. anegative voltage, may be applied to the bias lines 285 to create anelectric field (and hence a depletion region) across the p-n junction ofeach of the photodiodes 270 to enhance its collection efficiency for thecharges generated by incident light rays. The image signal representedby the array of photosensor cells 212 may be integrated by thephotodiodes 270 while their associated TFTs 271 are held in anon-conducting (off) state, for example, by maintaining the gate lines283 at a negative voltage via the gate driver circuits 228. The array ofphotosensor cells 212 may be read out by sequentially switching rows ofthe TFTs 271 to a conducting (on) state by means of the gate drivercircuits 228. When a row of the detector cells 222 is switched to aconducting state, for example by applying a positive voltage to thecorresponding gate line 283, collected charge from the photodiodes 270in those detector cells 222 may be transferred along data lines 284 andintegrated by the external charge amplifier circuits 286. The row maythen be switched back to a non-conducting state, and the process isrepeated for each row until the entire array of photosensor cells 212has been read out. The integrated signal outputs are transferred fromthe external charge amplifier circuits 286 to an analog-to-digitalconverter (ADC) 288 using a parallel-to-serial converter, such asmultiplexer 287, which together comprise read-out circuit 230.

This digital image information may be subsequently processed by imageprocessing system 34 to yield a digital image which may then bedigitally stored and immediately displayed on monitor 26, or it may bedisplayed at a later time by accessing the digital electronic memorycontaining the stored image. The flat panel DR detector 40 having animaging array as described with reference to FIG. 2 is capable of bothsingle-shot (e.g., static, radiographic) and continuous (e.g.,fluoroscopic) image acquisition.

FIG. 3 shows a perspective view of an exemplary prior art generallyrectangular, planar, portable wireless DR detector 300 according to anembodiment of DR detector 40 disclosed herein. The DR detector 300 mayinclude a housing 314 that encloses a multilayer structure comprisingthe photosensitive detector cells 22 of the DR detector 300. The housing314 of the DR detector 300 may include a continuous, rigid, radio-opaqueenclosure surrounding an interior volume of the DR detector 300. Thehousing 314 may comprise four orthogonal edges 318 and a bottom side 321disposed opposite a top side 322 of the DR detector 300. A top cover 312encloses the top side 322 which, together with the housing 314substantially encloses the multilayer structure in the interior volumeof the DR detector 300, and may be attached to the housing 314 to form aseal therebetween. The top cover 312 may be made of a material thatpasses x-rays 16 without significant attenuation thereof, i.e., aradiolucent material, such as a carbon fiber or plastic material.

With reference to FIG. 4, there is illustrated in schematic form anexemplary cross-section view along section A-A of the exemplaryembodiment of the DR detector 300 (FIG. 3). For spatial referencepurposes, one major surface of the DR detector 400 may be referred to asthe top side 451 and a second major surface may be referred to as thebottom side 452, as used herein. The multilayer imaging structure isdisposed within the interior volume 450 enclosed by the housing 314 andtop cover 312 and may include a scintillator layer 404 over thetwo-dimensional array 12 shown schematically as the device layer 402.The scintillator layer 404 may be directly under (e.g., directlyconnected to) the radiolucent top cover 312, and the device layer 402may be directly under the scintillator layer 404. Alternatively, aflexible layer 406 may be positioned between the scintillator layer 404and the top cover 312 as part of the multilayer structure to provideshock absorption. The flexible layer 406 may be selected to provide anamount of flexible support for both the top cover 312 and thescintillator layer 404, and may comprise a foam rubber type of material.

A substrate layer 420 may be disposed under the device layer 402, suchas a rigid glass layer upon which the device layer 402 is formed, andmay comprise another layer of the multilayer structure. Under thesubstrate layer 420 a radio-opaque shield layer 418 may be used as anx-ray blocking layer to help prevent scattering of x-rays 16 passingthrough the substrate layer 420 as well as to block x-rays 16 reflectedfrom other surfaces in the interior volume 450. Readout electronics,including the electronic scanning circuit 28, the electronic read-outcircuit 30, and the electronic bias circuit 32 (FIG. 1) may be formedco-planar with the device layer 402 or, as shown, may be disposed belowframe support member 416 in the form of integrated circuits electricallyconnected to printed circuit boards 424, 425. The frame support member416 is fixed to the housing 314 using frame support beams 422 to providesupport for the multilayer structure just described. The device layer402 is electrically connected to the readout electronics, 28, 30, 32,over a flexible connector 428 which may comprise a plurality offlexible, sealed conductors. X-rays 16 flux may pass through theradiolucent top panel cover 312, in the direction represented byexemplary x-rays 16, and impinge upon scintillator layer 404 wherestimulation by the high-energy x-rays 16, or photons, causes thescintillator layer 404 to emit lower energy photons as visible lightrays which are then received in the detector cells 222 of device layer402. The frame support member 416 may securely mount the multilayerstructure to the housing 314 and may further operate as a shock absorberby disposing elastic pads (not shown) between the frame support beams422 and the housing 314. Fasteners 410, such as screws, may be used tofixedly attach the top cover 312 to the housing 314 and create a sealtherebetween in the region 430 where they come into contact. In oneembodiment, an external bumper 412 may be attached along the orthogonaledges 318 of the DR detector 400 to provide additional shock-absorption.

FIG. 5 illustrates detector cell 222 connected to bias line bus 232,gate line 283, and data line 284, including a representation of aparasitic capacitance 276 between the source and drain of TFT 271. Theparasitic capacitance 276 couples the cathode of photodiode 270 to dataline 284. The parasitic capacitance 276 introduces a noise signal intothe data line 284 during an image readout operation by creating a lowimpedance path around TFT 271 even when the TFT 271 is in a highimpedance ‘OFF’ state. The charge storage capability of the photodiode270 is represented by the capacitance 275.

FIG. 6A illustrate an exemplary deleterious process occurring in thedetector cell 222 caused by effects of the extraneous signals. FIG. 6Aincludes two representative signal paths from photodiode 270. Firstsignal path 210 is connected from the cathode of photodiode 270 throughTFT 271 and out to data line 284 toward downstream read out circuitry,and is designed to carry DR detector image signals. The second signalpath 205 is a parasitic signal path that bypasses TFT 271 via aparasitic capacitance 276 which effectively couples the drain and sourceof the TFT 271. This first signal path 210 is created when TFT 271 isswitched to the low impedance ‘ON’ state using a signal on the gate line283 delivered by a gate driver connected to the gate line 283. Thisfirst signal path 210 is the designed signal conduction path and is usedduring an image readout operation to read out the charge level stored inthe photodiode 270 via its capacitive property, represented bycapacitance 275. The parasitic capacitance 276 may be referred to as aleakage capacitance that creates a low impedance conductive path fortime varying (non-DC) signals. An x-ray exposure period causes such atime varying signal due to the integration time wherein chargeaccumulates in the photodiode 270 via a photon generated photodiodecurrent, and so causes leakage into the data line 284 across theparasitic capacitance 276. An exemplary x-ray beam (photons) 215 may bereceived at the detector cell 222, initially impacting a scintillatorlayer 225 which, in response to the x-ray photons, emits light photons220. Light photons 220, in turn, impact photodiode 270 which, inresponse, generates charge carriers which are accumulated in thephotodiode 270 due to its intrinsic capacitance 275.

The graph of FIG. 6B illustrates a plot of various waveforms on itsvertical axis versus time, on its horizontal axis. Waveform A representsan x-ray pulse of finite duration received by the detector cell 222.While the x-ray pulse impacts the detector cell 222, charge carriersaccumulate in the photodiode 270 which is represented as a voltageramp-up in waveform B. The voltage ramp B may be represented as atime-varying voltage (dv/dt) and so causes a leakage across theparasitic capacitance 276, represented by the leakage current waveformC, through the second signal path 205 as described above. Thus, thetotal signal as measured on data line 284, represented by total signalwaveform D, during an x-ray pulse includes the sum of the pixel voltage(waveform B) plus the erroneous and extraneous leakage current ofwaveform C. As shown in the total signal waveform D at time t_(samp), anerror □□ is caused by the leakage current. The time varying voltageproduces leakage current over the second signal path 205 even when TFT271 is in the high impedance ‘OFF’ state. This leakage current is thesource of the extraneous data line signal caused by an x-ray exposureperformed concurrently with an image readout operation.

During image readout of any pixel, an extraneous leakage current signalwill be present on the data line and will equal the summed total of allother leakage currents in the pixels connected to the same data line,i.e. a column of pixels, by their parasitic capacitance 276. Thisresults in an image readout error that is present only during the timethat the pixel photosensor array receives x-ray fluence during an x-rayexposure. Image readout and x-ray exposure duration will rarely beequivalent, therefore, to insure that the image readout operationacquires all image data (photosensor charge), the image readoutoperation may be configured to extend longer in time than the x-rayexposure. This configuration will result in a part, but not all, of theimage readout time duration to be affected by the extraneous leakagecurrent.

FIG. 7 illustrates one embodiment of an image readout process 700wherein rows of pixels n 701, n+1 703, n+2 705, and so on, are each readout one at a time in sequence and stored into the image row buffer 707.FIG. 8 illustrates an embodiment of a modified image readout process 800using null row readouts 802, 804, 806, in a readout process to acquirecomplementary data sets that include image data information from imagereadouts 701, 703, 705 stored in image buffer 707, and extraneous signaldata information from null row readouts 802, 804, 806, stored in nullrow buffer 808. The buffers 707, 808, may include electronic memory forstoring a plurality of image data frames in different addressableportions of the electronic memory. Referring to FIG. 7 and FIG. 8, onemodified image readout process embodiment may include successive imagerow 701, 703, 705, readouts that are interleaved with null row 802, 804,806, readouts. Starting with read out of a particular image row n 701,the image data is digitized by A/D converters 288 (FIG. 2) and storedinto the image row buffer 707 at a memory location corresponding toimage row n 701. This image row readout is immediately followed by anull row 802 readout wherein the gate line 283 (FIG. 2) for thatparticular row of TFTs are turned off and any extraneous signal inducedonto their corresponding data lines is digitized by A/D converters 288and then stored into a null row buffer 808 at a memory locationcorresponding to image row n 701 of the image data. This interleavedprocess of alternate image row 701, 703, 705, readouts each followed bya null row 802, 804, 806, readout, respectively, may be termed a nullrow read operation and may be used to detect and capture extraneoussignals present on the data lines 284.

A null row read operation is similar to the standard image row readoutprocess except that during the null row read operation none of TFTs 271of data lines 284 are set to the ‘ON’ state. For example, the null rowread state may be achieved by keeping all row gate drivers 228 turnedoff while repeating the standard image row readout process. When a nullrow read process is performed the signal information acquired does notcontain image information from the pixels' photodiodes 270 but rathermay contain extraneous leakage signal information present on individualdata lines 284.

FIG. 9 illustrates a process 900 implementing complementary sets ofimage data frames 950, which set may include image data frames 951, 953,955, and 957, and null row data frames 960, which set may include nullrow data frames 961, 963, 965, and 967. Each set, 950, 960, of dataframes may include dark (or lag) image frames 955, 957, and dark (orlag) null row frames 965, 967, all data frames being acquired byperforming the interleaved readout procedure as described herein withrespect to FIG. 8. All the illustrated data frames 950, 960, may bestored in a storage buffer 923 comprising both the image row buffer 707and null row buffer 808. With respect to a horizontal axis representinga time duration 924, the storage buffer 923 may include additional dataframes captured during a time interval preceding a capture of the imageand null row data frames 951, 961, respectively, and after a capture ofthe image and null row data frames, 957, 967, respectively. Thus, a nullrow data frame captured in a preceding time interval may include a bandof extraneous signals such as illustrated in the null row data frame961, which may be used to infer that extraneous magnetic flux isaffecting operation of the DR detector. Such detection may be used totrigger a notification signal to the operator of the DR equipment toinvestigate potential sources of magnetic flux near the DR detector andto move such sources further from the DR equipment. As used herein, theterm “frame” or “data frame” represents the data captured by the arrayof photosensor cells 212 in a DR detector 40. Rows of pixel data areoriented vertically in the perspective of FIG. 9, wherein the first rowof pixel data is located to the left of each data frame, 951-957 and961-967, labeled “TOP”, and the last row of pixel data (i.e. bottom) islocated at the rightmost end of each data frame, 951-957 and 961-967, inthe perspective of FIG. 9. The rows of pixels in the DR detector arerepeatedly read out from top to bottom to generate the data frames951-957 and 961-967 as shown.

An x-ray source activation is illustrated as an exemplary 50 ms exposure903 beginning at first point in time 901 and continuing until the x-raysource is deactivated, or turned off, at a second point in time 909. Theamount of image data available to be read out from the array ofphotosensor cells 212 is represented by the graph 914. Points on thegraph 914 correspond to rows of pixels being read out from a DR detector40 providing the image data frames 950-960. The graph 914 indicatesthat, during the 50 ms exposure 903, an amount of image data availablein x-ray exposed pixels increases from about a zero percentage level atthe activation time point 901 to about a full 100% level 916 at thedeactivation time point 909, as indicated by the rising portion 915 ofthe graph 914. Because a number of rows of image data are being read outduring the rise time 915, each such row will have been read out withoutcomplete image data. Those rows being read out closer in time to thex-ray source activation point 901 will contain a smaller percentage ofthe full image data than the rows being read out closer in time to thex-ray source deactivation time point 909. Those rows being read outduring the time duration 905, after the x-ray source deactivation timepoint 909, will contain a full 100% of the image data as indicated bythe horizontal portion 916 of the graph 914. Note that during this fullreadout period 905 the last row of the DR detector's pixel array willhave been read out, at about the time 910, to complete the data frames951, 961, and that the DR detector readout will repeat, after about thetime 910, starting at the DR detector's first row (TOP) to generate thenext data frames 953, 963 (and repeating the readout for successive darkframes 955-957 and 965-967 as illustrated).

The falling portion 917 of the graph 914 represents an amount of data incorresponding rows of pixels that have not yet been read out. This maybe understood by noting that the row of pixels being read out from theDR detector 40 corresponding to the point in time at about 901 is thesame row of pixels being read out from the DR detector at the point intime at about 911 and the row of pixels being read out from the DRdetector corresponding to the point in time at about 909 is the same rowof pixels being read out from the DR detector at the point in time atabout 913. Thus, the rows of pixels that were read out during the timeperiod 903 contained partial image frame data (i.e., less than 100% dueto the active x-ray exposure not having been completed) wherein theremaining unread portion of image frame data from those rows of pixelsis read out during the time period 907. It may be noted that addingtogether the read out data from the rising and falling data portions915, 917, respectively, results in a full 100% read out of availableimage data with respect to that portion (or rows) of the image dataframe.

Activation of the x-ray source during the time period 903 causes anincrease in charge carriers in each exposed detector cell 222 of thearray of photosensor cells 212, which results in an induced time varyingvoltage in each exposed detector cell 222. As described herein, the timevarying voltage (dv/dt) generates a parasitic signal 919, in the graph918, in the pixels of the imaging array even when the readout TFTs 271are not turned on. This parasitic effect is shown in the null row databuffer image 961 wherein extraneous signals are generated during therising portion 915 of the graph 914 corresponding to the x-ray sourceactivation time 901.

A complete DR image may be obtained from the DR detector data framesobtained thus far by first adding together the image data frames 951 and953 which results in a 100% full read of the image frame datacorresponding to an x-ray exposure 903, and then subtracting from thattotal the null row image data represented in the null row data frame961. This combined image frame data may be comparable to therepresentation of the total data representing an x-ray exposure asdescribed with reference to graph D of FIG. 6B, and the null row dataframe may be comparable to the error data ε as shown in FIG. 6B.Subtraction of the error data (null row data frame 961) results in amore accurate DR image.

Referring now to FIGS. 10-11 there is illustrated an exemplary processof adding two image frames together as just described. Image data frame951 is added to image data frame 953 to obtain a total image data frame1101 for the x-ray exposure 903 which reconstructs all the image datacollected and stored in the image buffer. The various image frames 951,953, 961, may be stored in separate portions of the image buffer 923,and may be combined by addition or subtraction to replace in the buffermemory one of the combined images or, alternatively, the combined imagemay be stored in another portion of the image buffer. The extraneousnoise artifact may not be visible in the combined image of 1101 but canbe better observed in the magnified image wherein the parasitic signalcan be seen in the segment 1103 of the magnified combined image. This isthe portion of the total image data that is corrected via subtraction ofthe null row read data.

As exemplified herein, the image readout process for a DR detectorsystem may be performed after an x-ray expose-integration period hasoccurred. The purpose of the image readout operation is to acquire x-rayexposed patient image data from the DR detector's pixel array producedby the expose-integration process. The image information may besequentially read out from each pixel row of the detector array into aninternal image buffer, as described herein. The first exposed imagereadout may be immediately followed by a second non-exposed-integrationperiod, which may be performed during an interval when there is noincident x-ray radiation impinging the sensor array of the DR detector.Since no x-ray radiation is present during the non-exposed-integrationperiod, there is no new image information in the second read out image(e.g., lag image 955). However, because the first image readoutoperation leaves a small percentage of signal data remaining in thedetector pixel array, the second readout operation recovers this leftover signal data. The second read out image is typically referred to asimage lag or simply a dark image. These steps may be repeated to obtaina third non-exposed-integration operation image without x-ray radiationto obtain a second dark image frame. A post processing operation may beperformed on the set of acquired image frames, e.g., the exposed imagehaving one or two frames, the first non-exposed dark image frame and thesecond non-exposed dark image frame may be added together or otherwiseprocessed, to produce the final artifact free DR detector image.

The complementary set of image data and null row data may then beprocessed to determine the magnitude of any extraneous signal present onthe data lines during the image readout operation, as described herein.In one embodiment, when extraneous signal magnitude is above a certainthreshold, it may be compensated or removed from the image data by aprocess of combining (e.g., subtracting, weighting) the null row datafrom the image readout data. Subtracting the null row data from theimage readout data may reduce or remove extraneous signal noise from theimage readout data because Null Row data does not contain image datainformation from the pixel sensor array.

One caveat to such methods is that because the image readout operationis not perfectly simultaneous with the null row read operation, theremay be some error in any measured extraneous signal if the frequencycomponents of the extraneous signal are a prescribed amount greater than(e.g., twice) the readout sampling frequency of the null row readoperation. This condition violates the Nyquist sampling criterion andmay produce an erroneous aliased signal in the null row read data. Ifthese aliased signals are present then it may be difficult to remove theextraneous signals from the image data by subtracting the two imagesets.

When it is likely that extraneous signal frequencies are higher than thereadout sampling frequency an alternate method may be used. This methodimplements a series of null row read operations that are performedbefore an x-ray exposure process has been initiated and may be used todetect if there is extraneous signal noise from external low frequencymagnetic fields present on the data lines before the x-ray exposure andimage readout operation has started. In this case, successive null nowreads are performed and the digitized data line signals are stored intoa temporary row buffer similar to the buffers shown in FIG. 8. A realtime digital processing algorithm is then applied to the data from thenull row read operation to determine if any extraneous signal is presentby comparing a magnitude, or intensity, of the data obtained to athreshold. While the frequencies of external magnetic fields will mostlikely be greater than the null row read sampling frequency the aliasedsignals in the null row read data is not a concern because this methodonly needs to detect that an extraneous signal was present.

According to exemplary embodiments, null row read data may be used inseveral ways to detect, compensate for, reduce and prevent extraneoussignals from interfering with the standard image readout operation. Onenull row read process embodiment may be used to detect the presence ofextraneous signals (e.g., null row read data) before an x-ray exposureoccurs. As discussed previously, when the image readout operation isperformed concurrently with an x-ray radiation exposure operation anextraneous signal is impressed on all data lines in the pixel arraysensor. The magnitude of the extraneous signal on the individual dataline is dependent on the number of photons at each of the photodiodesites along the entire data line and this is dependent on the intensityof x-ray fluence impinging the scintillator at the photodiode sitesalong the length of the data line.

Exemplary digital processing algorithms to detect extraneous noise onthe data lines may be implemented in firmware and software using highspeed digital processing electronics, such as Field Programmable GateArrays (FPGAs) and CPUs, which are placed internal to the DR detectorsystem. If any extraneous signal is detected, this condition may becommunicated by the DR detector system hardware and software to theoperator through a visible/audible alert at the system console. Theoperator may then take preventative steps to remove the source ofmagnetic fields to avoid interference with the DR detector system imagereadout operation. This is especially useful for portable wireless DRdetector systems which, when used by mobile x-ray units, may be operatedin many different locations within a hospital or clinic.

When the image readout operation is complemented with an interleavednull row read operation according to embodiments described herein, meansmay be provided to perform the image readout operation during the x-rayexposure period and address or remove the inherent leakage current imageartifact produced by parasitic capacitances in the pixel array sensor.

In one exemplary embodiment, an error produced by extraneous leakagecurrent on the data line may be determined independent and separate fromthe image data by following an image row readout process with acorresponding null row read process. Since the leakage current ispresent on the data lines even when the TFTs are all turned off thisprovides a way to measure the extraneous leakage current right after ateach image row read. Since the leakage current is measured separatelyand independently, it may be subtracted from the image data in a postprocess operation. Further, because the extraneous leakage current ontothe data lines effectively remains at a constant level during the x-rayexposure, there is no danger that aliasing error will be present in thenull row read data.

Using the methods described herein makes possible the detection ofinterference caused by low frequency magnetic fields that would degradefinal image quality and thereby detecting and prevent such occurrence byalerting the operator of a DR detector system. Additionally, exemplarymethods and system embodiments described herein may provide a capabilityto perform image readout operations during the x-ray expose operationsand to be able to measure/monitor induced extraneous signals andcompensate for or remove induced extraneous signals from the image dataset to obtain final output images that are of clinical diagnosticquality.

As will be appreciated by one skilled in the art, the present inventionmay be embodied as a system, method, or computer program product.Accordingly, an embodiment of the present invention may be in the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, and other suitableencodings) or an embodiment combining software and hardware aspects thatmay all generally be referred to herein as a “circuit” or “system.”Furthermore, the present invention may take the form of a computerprogram product embodied in a computer-readable storage medium, withinstructions executed by one or more computers or host processors. Thismedium may comprise, for example: magnetic storage media such as amagnetic disk (such as a hard drive or a floppy disk) or magnetic tape;optical storage media such as an optical disc, optical tape, or machinereadable bar code; solid state electronic storage devices such as solidstate hard drives, random access memory (RAM), or read only memory(ROM); or any other physical device or medium employed to store acomputer program. The computer program for performing the method of thepresent invention may also be stored on computer readable storage mediumthat is connected to a host processor by way of the internet or othercommunication medium.

While the invention has been illustrated with respect to one or moreimplementations, alterations and modifications may be made to theillustrated examples without departing from the spirit and scope of theappended claims. In addition, while a particular feature of theinvention may have been disclosed with respect to only one of severalimplementations, such feature may be combined with one or more otherfeatures of the other implementations as may be desired and advantageousfor any given or particular function. The term “at least one of” is usedto mean one or more of the listed items may be selected. The term“about” indicates that the value listed may be somewhat altered, as longas the alteration does not result in nonconformance of the process orstructure to the illustrated embodiment. Finally, “exemplary” indicatesthe description is used as an example, rather than implying that it isan ideal. Other embodiments of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims.

1.-20. (canceled)
 21. An x-ray detector comprising: a plurality ofelectrically-chargeable photosensitive cells arranged in rows andcolumns; electric circuits connected to the plurality ofelectrically-chargeable photosensitive cells of each column and row toperform one or more readouts of charge contained in the plurality ofelectrically-chargeable photosensitive cells; an acquisition control andimage processing circuit programmed to: acquire one or more partialexposed image frames during the one or more readouts of charge containedin the plurality of electrically-chargeable photosensitive cells;acquire one or more dark frames during the one or more readouts ofcharge contained in the plurality of electrically-chargeablephotosensitive cells after acquiring the one or more partial exposedimage frames; and combine the one or more partial exposed image framesand the one or more dark frames.
 22. The x-ray detector of claim 21,wherein the acquisition control and image processing circuit is furtherprogrammed to detect an x-ray exposure by testing each row of theplurality of electrically-chargeable photosensitive cells for increasedimage signal intensity.
 23. The x-ray detector of claim 21, furthercomprising a first buffer and a second buffer, wherein the first bufferis configured to store the one or more partial exposed image frames, andwherein the second buffer is configured to store the one or more darkframes.
 24. The x-ray detector of claim 21, further comprising buffersfor storing image frames, wherein the buffers are configured to store anx-ray image comprising the one or more partial exposed image frames andthe one or more dark frames.
 25. The x-ray detector of claim 21, whereinthe acquisition control and image processing circuit is furtherprogrammed to acquire one or more null image frames during the one ormore readouts of charge contained in the plurality ofelectrically-chargeable photosensitive cells.
 26. The x-ray detector ofclaim 25, further comprising buffers for storing image frames, whereinthe buffers are configured to store an x-ray image comprising the one ormore partial exposed image frames and the one or more null image frames.27. The x-ray detector of claim 26, wherein the buffers are configuredto store an x-ray image comprising the one or more partial exposed imageframes, the one or more null image frames, and the one or more darkframes.
 28. The x-ray detector of claim 25, wherein the acquisitioncontrol and image processing circuit is further programmed to combinethe one or more partial exposed image frames and the one or more nullimage frames.
 29. The x-ray detector of claim 28, wherein theacquisition control and image processing circuit is further programmedto combine the one or more partial exposed image frames, the one or morenull image frames, and the one or more dark frames.
 30. The x-raydetector of claim 25, wherein the acquisition control and imageprocessing circuit is further programmed to determine if an extraneoussignal is present in any of the plurality of electrically-chargeablephotosensitive cells by comparing the one or more null image frames to athreshold.
 31. The x-ray detector of claim 21, further comprisingcontrollable row selecting switches to connect the electric circuits tothe electrically-chargeable photosensitive cells of each column toprovide the one or more readouts of charge contained in the plurality ofelectrically-chargeable photosensitive cells.
 32. The x-ray detector ofclaim 31, wherein the one or more readouts of charge contained in theplurality of electrically-chargeable photosensitive cells is performedone row at a time.
 33. The x-ray detector of claim 31, wherein theacquisition control and image processing circuit is further programmedto acquire the one or more partial exposed image frames by exposing theelectrically-chargeable photosensitive cells to radiographic radiationduring the one or more readouts of charge contained in the plurality ofelectrically-chargeable photosensitive cells and to acquire the one ormore dark frames by not exposing the electrically-chargeablephotosensitive cells to radiographic radiation during the one or morereadouts of charge contained in the plurality of electrically-chargeablephotosensitive cells.
 34. A method comprising: image scanning a twodimensional array of pixels to determine a first image charge leveltherein; exposing the two dimensional array of pixels to a pulse ofradiographic radiation during the step of image scanning; imagerescanning the two dimensional array of pixels to determine a secondimage charge level therein; and combining the determined first imagecharge level with the determined second image charge level of the twodimensional array of pixels.
 35. The method of claim 34, furthercomprising: recording in electronic memory the determined first imagecharge level of the two dimensional array of pixels; and recording inthe electronic memory the determined second image charge level of thetwo dimensional array of pixels, and wherein the step of combiningincludes combining the recorded first image charge level with therecorded second image charge level of the two dimensional array ofpixels.
 36. The method of claim 34, wherein the step of exposingincludes generating in the two dimensional array of pixels at least aportion of the determined first image charge level and at least aportion of the determined second image charge level.
 37. The method ofclaim 34, further comprising: null scanning the two dimensional array ofpixels to determine a null image charge level therein, wherein the stepof image scanning includes reading out a portion of the two dimensionalarray of pixels while a corresponding gate line is turned on, andwherein the step of null scanning includes reading out a portion of thetwo dimensional array of pixels while the corresponding gate line isturned off.
 38. The method of claim 37, further comprising subtractingthe determined null image charge level in the two dimensional array ofpixels from the combined first and second image charge levels in the twodimensional array of pixels.
 39. The method of claim 37, wherein thestep of image scanning the two dimensional array of pixels comprisessequentially image scanning each row of pixels individually, and whereinthe step of null scanning the two dimensional array of pixels comprisessequentially null scanning each row of pixels individually immediatelyafter image scanning each row of pixels individually.
 40. The method ofclaim 34, further comprising: dark scanning the two dimensional array ofpixels to determine a dark image charge level therein; and combining thedetermined dark image charge level in the two dimensional array ofpixels with the combined first and second image charge levels in the twodimensional array of pixels, wherein the step of dark scanning includesnot exposing the two dimensional array of pixels to any radiographicradiation during the step of dark scanning.
 41. The method of claim 37,further comprising: dark scanning the two dimensional array of pixels todetermine a dark image charge level therein; and combining thedetermined dark image charge level in the two dimensional array ofpixels, the determined null image charge level in the two dimensionalarray of pixels, and the combined first and second image charge levelsin the two dimensional array of pixels, wherein the step of darkscanning includes not exposing the two dimensional array of pixels toany radiographic radiation during the step of dark scanning.